In certain applications, it is advantageous for a field-deployed unit to have knowledge of its orientation. For example, a unit may require knowledge of its own orientation relative to a target, or an array of units may require knowledge of their respective orientations relative to each other and/or relative to a target. In a data share process, for example, multiple units communicate with each other in order to effectively track a target of interest, or in order to determine which unit can most effectively engage a target. Efficient processing requires orientation estimates by each unit involved in the tracking process. In another function, referred to as control station communication, each individual unit communicates with a central computer to provide an operator of the central computer with updated tracking information so that the operator can plan an effective solution. Again, in this case, orientation information for each unit involved is critical for accurate engagement.
While the popular global positioning system (GPS) provides an accurate accounting of latitude, longitude, and altitude of a unit, as well as an accurate time reference, the orientation of a unit cannot be derived solely from the received GPS data. Accordingly, magnetic and electronic compasses remain as popular mechanisms for providing orientation information. When properly calibrated, such compasses commonly achieve orientation readings to within a tolerance of ±2°.
A magnetic compass detects the horizontal direction of the earth's magnetic field. Using this reference, a unit can derive its orientation. However, the accuracy of a magnetic compass is limited by environmental issues, such as hard and soft iron effects in the surrounding landscape, and variations in the earth's magnetic field. In addition, the magnetic fields generated by nearby system electronics can further interfere with accurate readings. Furthermore, a magnetic compass requires periodic calibration, which can be an expensive operation when the unit is in long-term storage or when the unit is deployed in the field.
The electronic compass compensates for the hard and soft iron effects by using specific calibration algorithms. Soft iron effect calibration is quite complicated and requires an initial calibration procedure when a unit is deployed in the field. The initial calibration can be easily disturbed if the unit is moved, and a complete system recalibration is required every few months. Such recalibration is often times impractical or impossible for field-deployed units. Furthermore, electronic compasses are sensitive to temperature, especially outside the range of −40 C to 80 C.